Guide to Emissivity (Part Two)

Resources

Last month, we discussed thermal radiation and material properties and the thermal nature of materials. We also looked at the emissivity of real objects using a stainless steel block. This month, we will discuss other characteristics demonstrated by the steel block example.

Emissivity, the Variable Variable

Using our steel block example from last month, we will discuss another significant phenomena. We will take our unpainted metal block and drill three holes part way into the body. All three holes are 1/8-in diameter. The first is 1/8-in deep, the second is ¼-in deep and the third is 3/8-in deep. Bake the block for another three hours, then remove the block and observe it again with the camera (see Figure IR4).

Figure IR4

Interestingly, the hot block surface appears to be about 84-deg F, and now appears to have three hot spots. The 1/8-in deep hole appears to be 106-deg F. The 1/4-in deep hole appears to be 112-deg F; and the 3/8-in deep hole appears to be 125-deg F.

We know that the metal block is actually about 175-deg F (measured by a thermocouple), and the surface finish is uniform and has an emissivity of approximately 0.12. The reason the temperature appears to be higher in the holes is that a hole in a body enhances the emissivity. The greater the depth/diameter ratio of the hole, the greater the emissivity enhancement. By adjusting the emissivity on the thermal imager to match the actual temperature at each hole, we find that the emissivity appears to be 0.25 for the 1/8-in deep hole. The emissivity of the 1/4-in deep hole appears to be 0.35, and the 3/8-in deep hole appears to have an emissivity of 0.45.

We need another piece of electrical equipment to see why this is an extremely important effect.

Figure 5. Three-phase power disconnect

Figure IR5. Corresponding thermal image.

Emissivity and Electrical Equipment

Figures 5 and IR5 show another power disconnect with the conductors bolted in place using allen head bolts. The corresponding infrared image shows a hot connection on the middle phase. Notice the apparent hot spot in the hot allen socket head. The well of the bolt head appears hotter primarily because the well illustrates the blackbody effect of a hole.

In manufacturing processes, steel or aluminum rolls are often used to heat or cool a material such as in paper or plastic film processing. These rolls are usually polished metal surfaces, and it is important to understand the thermal profile since the manufacturing process depends on thermal uniformity across the rolls. The temperature of these rolls can be difficult to measure with a thermal imager because they have low emissivities. However, there are often points where the material passes between two rolls. The tangent point between two rolls also tends to simulate the blackbody effect, allowing for effective temperature measurement in an otherwise difficult situation.

This effect is illustrated in common electrical equipment as well (see Figure 6). In this case, we have another power disconnect with knife blade switches. This type of switch utilizes shiny metal blades, and the proximity of the blades with narrow gaps simulates the blackbody effect for greatly improved effective emissivity.

The important message here is to develop your understanding of apparent and actual temperature measurements. Actual temperature measurement requires an intimate understanding of physics, heat transfer and characteristics of materials.

Figure 6. Power disconnect with knife blade connectors.

Figure IR6. Corresponding thermal image.

Qualitative Versus Quantitative Infrared Thermography

Emissivity difficulties are not a barrier to effectively using infrared thermography for predictive maintenance (PdM). ASTM standards exist to guide thermographic PdM inspections. These standards describe the use of thermal imagers for qualitative and quantitative infrared inspections.

Quantitative infrared inspections require determining each component's emissivity to make accurate temperature measurements possible. This practice may not always be necessary for routine inspections, unless the exact temperature value is needed for long term tracing. Qualitative methods, in contrast, allow you to leave the emissivity at 1.0 and evaluate the equipment on a relative basis. Has it changed, or is it different? The basis for qualitative evaluation is comparing similar equipment under similar loads.